First pass on energy savings

We’re getting ourselves ready to build some software to help predict whole house fan energy savings. In the meantime, I put together this spreadsheet, which I think is a pretty good approximation for the coastal areas of California.

Potential Whole House Fan Energy Savings per Year for California





Input Energy cost arrow $0.24 Dollars per KWH
Application High Speed Watts Low Speed Watts Energy Savings * (per Year)
2.5WHF 288 250 $777.89
2.5eWHF 200 43 $826.70
Air Conditioning 2808 na $0
* Assumptions are that the cooling season is 5 months, the WHF is run 10 hours/day, 40% use is at low speed, an average deltaT at night is 10º , and that all WHF cooling offsets AC use.

4 thoughts on “First pass on energy savings”

  1. I agree with your first two assumptions, but wouldn’t the majority of the time (overnight) be spent on low-speed? Also, what’s the “deltaT at night” — the difference in temperature between the outside air and the inside air? I see that it’s an average, i.e., you’re accounting for the fact that both temperatures are going to change during the night. I’m curious what impact that has on your calculation.

    Also, do typical fans (e.g. your 2.5WHF) really use 90% of the ‘full-power’ even at low speeds?

    What I’d like to see is a pair of graphs showing the change in inside-temp, outside-temp, and energy use during the course of a summer day (full 24 hours) for both a traditional A/C setup and a typical whole-house fan setup.

  2. As to whether the WHF would usually run at low speed; that will depend upon how you size your whole house fan. Ideally, yes, a whole house fan should be sized to provide quick relief (lots of air), but be able to throttle down for (most) nights at a very efficient rate. For this analysis, we made some assumptions that we believe to be reasonably accurate, like the delta T between inside and outside is about 10 degrees. To jump ahead to the last part of your comment, that would involve doing an 24/7/365 energy analysis of your house. The software is there (might need some tweaking for free cooling), but to be accurate you would have to “describe” your structure, insulation, windows, doors, outside shading to the software. There will be some cooling potential/energy analysis software that we are working on coming out shortly.
    Yes, most PSC (the usual type) motors do not unload well at all. Many, if not most motors are running at 30%-40% efficiency, ie. for every 1,000 watts of electrical energy in, 300-400 watts of mechanical work comes out.. It’s a big problem opportunity. 🙂

  3. The “energy savings calculator” says “our Energy Calculator predicts the energy savings of your Whole House Fan . . . ,” and gives a number; is that number annual savings, monthly savings or something else? Not clear.

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